Compound semiconductor light-emitting diode and method for fabrication thereof

ABSTRACT

A compound semiconductor light-emitting diode includes a light-emitting layer ( 133 ) formed of aluminum-gallium-indium phosphide, a light-emitting part ( 13 ) having component layers individually formed of a Group III-V compound semiconductor, a transparent supporting layer ( 14 ) bonded to one of the outermost surface layers ( 135 ) of the light-emitting part ( 13 ) and transparent to the light emitted from the light-emitting layer ( 133 ), and a bonding layer ( 141 ) formed between the supporting layer ( 14 ) and the one of the outermost surface layers ( 135 )of the light-emitting part ( 13 ) containing oxygen atoms at a concentration of 1×10 20  cm −3  or less.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is an application filed under 35 U.S.C. §111(a)claiming the benefit pursuant to 35 U.S.C. §119(e)(1) of the filingdates of Provisional Application No. 60/700,346 filed Jul. 19, 2005 andJapanese Patent Application No. 2005-197009 filed Jul. 6, 2005 pursuantto 35 U.S.C. §111(b).

TECHNICAL FIELD

This invention relates to a compound semiconductor light-emitting diodeincluding a light-emitting layer formed of aluminum-gallium-indiumphosphide ((Al_(X)Ga_(1-X))_(Y)In_(1-Y)P wherein 0≦X≦1 and 0<Y≦1) and,as well, having component layers individually provided with alight-emitting part formed of a Group III-V compound semiconductor and atransparent support layer bonded to one of the outermost surface layersof the light-emitting part and transparent to the light emitted from thelight-emitting layer and to a method for the fabrication thereof

BACKGROUND ART

As a light-emitting diode (LED) that emits a visible light in a red,orange, yellow or yellowish green color, the compound semiconductor LEDprovided with a light-emitting layer formed of aluminum-gallium-indiumphosphide ((Al_(X)Ga_(1-X))_(Y)In_(1-Y)P wherein 0≦X≦1 and 0<Y≦1) hasbeen known heretofore. In the LED of this configuration, generally thelight-emitting part provided with a light-emitting layer formed of(Al_(X)Ga_(1-X))_(Y)In_(1-Y)P (0≦X≦1, 0<Y≦1) is optically nontransparentto the light emitted from the light-emitting layer and is formed on thesubstrate of a material, such as gallium arsenide (GaAs), which has noappreciably high strength.

Recently, therefore, for the purpose of obtaining a visible LEDexhibiting enhanced luminance and with the object of further enhancingthe mechanical strength as a device, a technique for configuring ajunction-type LED by removing a nontransparent substrate material, suchas GaAs, and thereafter bonding anew a support layer formed of amaterial transparent to the emitted light and excellent in mechanicalstrength more than ever has been developed. The transparent supportinglayer, such as a Group III-V compound semiconductor crystal substrate,for example, is ordinarily disposed as bonded to the surface of a buffer(barrier) layer and not to the surface exposed by the removal of anontransparent substrate, such as a layer forming a light-emitting part.

As means for effecting adhesion of a transparent supporting layer, thefollowing methods (1) to (5) have been known heretofore.

-   (1) A method for directly bonding the supporting layer to a    semiconductor layer while applying pressure thereto at an elevated    temperature of several hundred degrees (refer to Japanese Patent No.    3230638).-   (2) A method for effecting bonding by a means called wafer bonding    (refer to JP-A HEI 6-302857).-   (3) A method utilizing a transparent adhesive substance, such as    epoxy resin (refer to JP-A 2002-246640).-   (4) A method for bonding a semiconductor layer and the transparent    supporting layer through a transparent electrically conductive thin    film, such as of an indium-tin complex oxide (ITO) (refer to    Japanese Patent No. 2588849).-   (5) A method comprising the steps of mirror-polishing both a    semiconductor layer and a supporting layer, bonding the two layers    after removal of defiling matter and heat-treating the bonded layers    (refer to JP-A 2001-57441).

The technical means of (1) which attempts to bond a transparentsupporting layer directly to the surface of a semiconductor, however,necessitates elevation of temperature to a high level of 600° C. or moreand application of pressure as well (“Semiconductors and Semimetals,”Vol. 48, edited and written by G. B. Stringfellow and M. George Craford(published in 1997 by Academic Press (U.S.A.)), refer to pp. 196-206).An attempt to bond a transparent supporting layer under such conditionsof high temperature and high pressure leads to inducing a disadvantagein easily inflicting a crystal defect on the supporting layer becausestress is exerted, for example, on a Group III-V compound semiconductorlayer which possesses brittleness. When the surface of the Group III-Vcompound semiconductor layer to be bonded, for example, is not flat, thepressure is unevenly applied to the layer, with the result that thebonded layers will frequently form such union as is inferior in qualityand deficient in strength. Further, the disadvantage of the conventionalmeans of bonding under the conditions of high temperature and highpressure consists in the fact that an effort to bond a supporting layerwhich reveals difference in thermal expansion coefficient results ininducing a warp due to mechanical stress and eventually entailingoccurrence of crystal defects in a large amount in the interface ofunion.

The bond produced by the wafer bonding means of (2), being formed with atransparent supporting layer and a Group III-V compound semiconductorlayer, for example, is at a disadvantage in suffering the presence of anoxide film on the surfaces of the layers being bonded or thecontamination caused by a pasting agent used for bonding to degradestrength of union and aggravate electric resistance in the interface ofunion. In the LED, therefore, the expected decrease of the forwardvoltage (Vf), for example, encounters a hindrance.

Meanwhile, the bonding means of applying or inserting a viscous adhesivematerial, such as epoxy resin of (3) or a transparent electricallyconductive thin film of (4), between the supporting layer and thesemiconductor layer being bonded, in spite of being able to lower thetemperature necessary for the bonding, is at a disadvantage in sufferingthe intervention of a foreign material, such as the epoxy resin layer,in the interface of union to inflict the stress due to the difference inthermal expansion coefficient on the Group III-V compound semiconductorlayer, and consequently induce the leak of the electric current foroperating the device (device operating current) via the crystal grainboundaries generated by the stress, and prevent the junction-typecompound semiconductor LED abounding in reverse voltage from beingobtained with fully satisfactory stability.

Particularly, in the compound semiconductor LED, when a transparentsupporting layer gaining in thickness in the direction of flow of thedevice operating current and additionally performing an action ofdiffusing the device operating current throughout the whole mass of thelight-emitting layer is disposed as bonded to the LED, the difference inthermal expansion coefficient between the transparent supporting layerand the adhesive material induces separation of the transparentsupporting layer from the adhesive material conspicuously. This bondingmeans, as compared with the means of directly bonding them togetherwithout daring use of an adhesive material, poses a problem that thetransparent supporting layer cannot be bonded to the light-emitting partwith fully satisfactory strength and the junction-type LED cannot befabricated with fully satisfactory stability.

Then, the surface cleaning of (5) which resorts to mirror polishing,removal of defiling matter or the like requires a highly advancedcleaning technique and subsequently necessitates an environment ofcleanliness exceptionally high enough to avoid re-contamination andrenders stable fabrication difficult to achieve. Further, adjustment ofthe environment entails a problem of adding to the burden of cost.

This invention has been proposed in view of the foregoing state ofaffairs and is aimed at providing a compound semiconductorlight-emitting diode capable of suppressing the occurrence of a crystaldefect without exerting stress on a light-emitting part, enhancing thebonding strength between the light-emitting part and a supporting layer,further decreasing electric resistance in the interface of union andimproving the forward voltage (Vf), heightening also the reversevoltage, and realizing enhancement of luminance and at providing amethod for the fabrication thereof.

DISCLOSURE OF THE INVENTION

With a view to accomplishing the object mentioned above, the firstaspect of the invention is directed to a compound semiconductorlight-emitting diode comprising a light-emitting layer formed ofaluminum-gallium-indium phosphide ((Al_(X)Ga_(1-X))_(Y)In_(1-Y)P wherein0≦X≦1 and 0 <Y≦1), component layers individually having a light-emittingpart formed of a Group III-V compound semiconductor, a transparentsupporting layer bonded to one of the outermost surface layers of thelight-emitting part and transparent to the light emitted from thelight-emitting layer, and a bonding layer formed between the supportinglayer and the one of the outermost surface layers of the light-emittingpart and containing oxygen atoms at a concentration of 1×10²⁰ cm⁻³ orless.

In the second aspect of the invention, besides fulfilling theconfiguration of the first aspect of the invention, the bonding layerformed between the supporting layer and one of the outermost surfacelayers of the light-emitting part contains carbon atoms at aconcentration of 1×10²⁰ cm⁻³ or less.

In the third aspect of the invention, besides fulfilling theconfiguration of the first or second aspect of the invention, the one ofthe outermost surface layers of the light-emitting part has a differentlattice constant from the other component layers of the light-emittingpart and has a thickness of 0.5 μm or more and 20 μm or less.

In the fourth aspect of the invention, besides fulfilling theconfiguration of any one of the first to third aspects of the invention,the supporting layer and the one outermost surface layer of thelight-emitting part are both formed of gallium phosphide (GaP).

In the fifth aspect of the invention, besides fulfilling theconfiguration of the fourth aspect of the invention, the bonding layerhas a nonstoichiometric composition, represented by the formulaGa_(X)P_(1-X) wherein 0.5<X<0.7.

In the sixth aspect of the invention, besides fulfilling theconfiguration of any one of the first to fifth aspects of the invention,the bonding layer has a thickness of 0.5 nm or more and 5 nm or less.

In the seventh aspect of the invention, besides fulfilling theconfiguration of any one of the first to sixth aspects of the invention,a first electrode is formed on the other outermost surface layer of thelight-emitting part, a second electrode is formed on the surface of thesupporting layer, the first electrode comprises a transparent,electrically conductive film formed on an ohmic electrode, and a bondingelectrode is formed on the transparent, electrically conductive film.

The eighth aspect of the invention is directed to a method for thefabrication of a compound semiconductor light-emitting diode including alight-emitting layer formed of aluminum-gallium-indium phosphide((Al_(X)Ga_(1-X))_(Y)In_(1-Y)P wherein 0≦X≦1 and 0<Y≦1) and, as well,having component layers individually possess a light-emitting partformed of a Group III-V compound semiconductor and a transparentsupporting layer bonded to one of the outermost surface layers of thelight-emitting part and transparent to the light emitted from thelight-emitting layer, which method comprises the steps of growing thecomponent layers on a substrate to form the light-emitting part,polishing the light-emitting part by mirror-polishing the surfaces ofthe outermost surface layers of the light-emitting part till averageroughness of 0.3 nm or less, preparing the supporting layer separatelyof the light-emitting part, irradiating at least either of the outermostsurface of the light-emitting part and the surface of the supportinglayer in a vacuum with atoms or ions possessing an energy of 50 eV ormore, and bonding the surface of the outermost layer of thelight-emitting part and the surface of the supporting layer.

In the ninth aspect of the invention, besides fulfilling theconfiguration of the eighth aspect of the invention, the surface of thesupporting layer is mirror-polished to 0.3 nm or less in theroot-mean-square value.

In the tenth aspect of the invention, besides fulfilling theconfiguration of the eighth or ninth aspect of the invention, the atomor ion irradiated in the irradiating step is one member selected fromthe group consisting of a hydrogen atom (H), a hydrogen molecule (H₂)and a hydrogen ion (H⁺).

In the eleventh aspect of the invention, besides fulfilling theconfiguration of the eighth or ninth aspect of the invention, the atomor ion in the irradiating step is one or more members selected from thegroup consisting of helium (He), neon (Ne), argon (Ar) and krypton (Kr).

In the twelfth aspect of the invention, besides fulfilling theconfiguration of any one of the eighth to eleventh aspects of theinvention, the bonding step is performed at room temperature or more and100° C. or less.

In the thirteenth aspect of the invention, besides fulfilling theconfiguration of the eighth aspect of the invention, at least either ofthe surface of the outermost layer of the light-emitting part and thesurface of the supporting layer is subjected to a wet or dry etchingtreatment.

In the fourteenth aspect of the invention, besides fulfilling theconfiguration of any one of the eighth to thirteenth aspects of theinvention, the method further comprises a step of removing the substratefrom the light-emitting part.

According to this invention, since the concentration of oxygen atoms inthe bonding layer formed between the supporting layer and one of theoutermost surface layers of the light-emitting part is set at 1×10²⁰cm⁻³ or less, the supporting layer and the one outermost surface layerof the light-emitting part can be bonded fast. Further, the introductionof a crystal defect into the light-emitting part can be suppressed andconsequently the uncalled-for addition to the electric resistance in thedirection of flow of the device operating current can be avoided. As aresult, a compound semiconductor light-emitting diode that shows a lowforward voltage (Vf) and a small leak current via crystal defect as welland abounds in reverse voltage, for example, can be configured.

According to this invention, since one of the outermost surface layersof the light-emitting part has a different lattice constant from theother component layers of the light-emitting part and has a thickness of0.5 μm or more and 20 μm or less, the infliction of distortion on theother component layers of the light-emitting part can be prevented bythe function of the outermost surface layer discharged toward allayingstress.

According to this invention, since the supporting layer and one of theoutermost surface layers of the light-emitting part are both formed ofgallium phosphide (GaP), the bonding strength can be enhanced by theequality of material and the transmission of the light emitted from thelight-emitting part can be realized, with the result that a compoundsemiconductor light-emitting diode that excels in the efficiency ofextraction of the emitted light to the exterior will be provided.

According to this invention, since the gallium phosphide (GaP) in one ofthe outermost surface layers of the light-emitting part is made toassume a nonstoichiometric composition, represented by formulaGa_(X)P_(1-X)(0.5<X<0.7), the introduction of distortion into thelight-emitting part can be avoided and the supporting layer and one ofthe outermost surface layers of the light-emitting part can be bondedfast.

According to this invention, since the bonding layer possesses acomposition different from gallium phosphide (GaP) and a thickness of0.5 nm or more and 5 nm or less, the supporting layer and one of theoutermost surface layers of the light-emitting part can be bonded fast.

According to this invention, since a first electrode is formed on theother outermost surface layer of the light-emitting part, a secondelectrode is formed on the surface of the supporting layer, the firstelectrode is composed of a transparent, electrically conductive filmformed on an ohmic electrode and a bonding electrode is formed on thetransparent, electrically conductive film, a compound semiconductorlight-emitting diode of high luminance can be easily provided.

According to this invention, since the surface of the outermost surfacelayer of the light-emitting part is mirror-polished to average roughnessof 0.3 nm or less and, as well, the surface of the outermost surfacelayer of the light-emitting part and the surface of the supporting layerare bonded by dint of the irradiation of at least either of the surfaceof the outermost surface layer of the light-emitting part and thesurface of the supporting layer in vacuum with an atom or ion possessingenergy of 50 eV or more, a strong bond can be formed by mutually bondingthe polished flat surfaces, the surfaces being bonded can be activatedand an impurity layer and a polluting layer existing on the surfacesbeing bonded can be removed by irradiating the flat surfaces with theatom or ion, and consequently the transparent supporting layer can bestrongly bonded directly to the light-emitting part.

According to this invention, since the surface of the supporting layeris mirror-polished to 0.3 nm or less in the root-mean-square value, thesurface of the supporting layer can be made to gain further in flatnessand the bond thereof to the outermost surface layer of thelight-emitting part can be made to gain in strength.

According to this invention, since the bonding in the bonding step iscarried out at room temperature or more and 100° C. or less, thetransparent supporting layer can be directly bonded to thelight-emitting part without entailing uncalled-for impartation ofdistortion and a compound semiconductor light-emitting diode of highluminance can be fabricated stably.

Besides, according to this invention, since at least either of thesurface of the outermost surface layer of the light-emitting part andthe surface of the supporting layer is subjected to a wet or dry etchingtreatment, the surface can be further improved in flatness and can becleaned as well by the removal of foreign matter or polluting matteradhering thereto.

Furthermore, according to this invention, since the transparentsupporting layer is directly bonded to the light-emitting part andthereafter the substrate previously used for the formation of thelight-emitting part is removed, the absorption of the light emitted fromthe light-emitting part by the substrate can be avoided and, as aresult, a compound semiconductor light-emitting diode of high luminancecan be fabricated.

The above and other objects, characteristic features and advantages ofthe present invention will become apparent to those skilled in the artfrom the description to be made herein below with reference to theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view illustrating a schematic plan view of thesemiconductor light-emitting diode manufactured in the example.

FIG. 2 is a cross section taken through FIG. 1 along line II-II.

FIG. 3 is a schematic view of the component layers of an epitaxiallystacked structure to be used in a semiconductor light-emitting diode.

FIG. 4 is a schematic view of the component layers of an epitaxiallystacked structure in a state having a supporting layer bonded thereto.

FIG. 5 is a schematic plan view illustrating the structure of an LEDlamp.

FIG. 6 is a schematic cross section illustrating the structure of theLED lamp.

FIG. 7 is a schematic plan view illustrating the shape of an ohmicelectrode.

FIG. 8 is a schematic plan view illustrating the shape of another ohmicelectrode.

FIG. 9 is a schematic plan view illustrating the shape of a stillanother ohmic electrode.

FIG. 10 is a schematic plan view illustrating the shape of a yet anotherohmic electrode.

FIG. 11 is a schematic plan view illustrating the shape of a furtherohmic electrode.

BEST MODE OF CARRYING OUT THE INVENTION

The light-emitting part according to this invention is a region whichcontains a light-emitting layer formed of (Al_(X)Ga_(1-X))_(Y)In_(1-Y)P(0≦X≦1, 0<Y≦1) and discharges the function of emitting light. Thelight-emitting layer can be formed of (Al_(X)Ga_(1-X))_(Y)In_(1-Y)P;(0≦X≦1, 0<Y≦1) in the form of conduction of either the n-type or thep-type. Though the light-emitting layer may be in either of thestructures of single quantum well (SQW) and multiquantum well (MQW), itis preferably in the MQW structure for the purpose of obtaining anemitted light excelling in monochromaticity. The barrier layerconstituting a quantum well (QW) structure and the composition of(Al_(X)Ga_(1-X))_(Y)In_(1-Y)P (0≦X≦1, 0≦Y≦1) constituting a well layerare so decided that the quantum level destined to fix an expectedwavelength of the emitted light may be formed in the well layer.

The light-emitting part, most preferably for the sake of acquiring lightemission of high intensity, is formed in the so-called double hetero(DH) structure which is composed of the light-emitting layer and cladlayers disposed as opposed to the opposite sides of the light-emittinglayer with the object of confining in the light-emitting layer thecarrier and the light emission responsible for radiation recombination.The clad layers are preferably formed of a semiconductor material havinga larger forbidden band width and a higher refractive index than thecompound (Al_(X)Ga_(1-X))_(Y)In_(1-Y)P (0≦X≦1, 0<Y≦1) forming thelight-emitting layer. As regards the light-emitting layer which isformed of (Al_(0.4)Ga_(0.6))_(0.5)In_(0.5)P capable of emitting ayellowish green color with a wavelength of about 570 nm, for example,the clad layers are formed of (Al_(0.7)Ga_(0.3))_(0.5)In_(O.5)P (Y.Hosakawa et at., J. Crystal Growth, 221 (2000), pp. 652-656). It ispermissible to have interposed between the light-emitting layer and theclad layers such intermediate layers as are intended to changemoderately the discontinuity of a band between these layers. In thiscase, it is preferable to have the intermediate layers formed of asemiconductor material that possesses a forbidden band widthintermediating between the light-emitting layer and the clad layers.

In the compound semiconductor light-emitting diode comprising alight-emitting layer formed of aluminum-gallium-indium phosphide((Al_(X)Ga_(1-X))_(Y)In_(1-Y)P wherein 0≦X≦1, 0≦Y≦1), component layersindividually having a light-emitting part formed of a Group III-Vcompound semiconductor and a transparent supporting layer bonded to oneof the outermost surface layers of the light-emitting part andtransparent to the light emitted from the light-emitting layer, thisinvention contemplates setting the concentration of oxygen atoms in abonding layer formed between the supporting layer and one of theoutermost surface layers of the light-emitting part at 1×10²⁰ cm⁻³ orless and the concentration of carbon atoms in the bonding layer at1×10²⁰ cm⁻³ or less.

What is essential first of all in the case of bonding the transparentsupporting layer to the outermost surface layer of the Group III-Vcompound semiconductor layer, i.e. a layer partaking in the constructionof the light-emitting part, is the surface treatment of the outermostsurface layer on the side of the light-emitting part about to have thesupporting layer bonded thereto. Particularly, a means of surfacetreatment that is intended to remove an oxide film is necessary. As amethod of surface treatment proper for gallium phosphide (GaP), forexample, the method of wet treatment that resides in immersing a givensurface in hydrofluoric acid (HF) may be cited. Also, the method whichconsists in immersing a surface in a mixed liquid containing sulfuricacid (H₂SO₄) or phosphoric acid (H₃PO₄) and subsequently giving thesurface a final surface treatment with a solution containing HCl may becited. The dry etching method using an inert gas, such as argon (Ar), isavailable as a means for removing an oxide film existing on the surfaceof GaP. GaP, for example, can be investigated by determining theexistence of a signal caused by the chemical bond of gallium (Ga) withoxygen (O) or of phosphorus (P) with oxygen by means of analysis, suchas infrared (IR) absorption spectroscopy or Electron Spectroscopy forChemical Analysis (ESCA).

The bonding strength between the outermost surface layer of thelight-emitting part and the transparent supporting layer destined to bebonded to the surface thereof depends very conspicuously on theconcentration of oxygen in the bonding layer. The adhesive strengthdecreases in accordance as the concentration of oxygen atoms in thebonding layer increases. In order that the individual devices beingseparated by cutting may withstand the impact of chipping withoutinducing separation from the bonding layer, the concentration of oxygenatoms in the bonding layer is preferably set at 1×10²⁰ atoms/cm³ orless. The treatment of the surface of the outermost surface layer of thelight-emitting part that is performed by the method described abovemanifests the effect of stably lowering the concentration of oxygen inthe bonding layer to below the concentration of atoms. If the oxygenatoms are present in the bonding layer in a large amount exceeding theconcentration of 1×10²⁰ atoms/cm³, the bonding strength will decreaseconspicuously. For this reason, the separation of individual devices bycutting, for example, will be at a disadvantage in suffering thetransparent supporting layer to peel off the outermost surface layer ofthe light-emitting part, preventing the devices from being normallyfabricated, and so forth.

Further, when the transparent supporting layer of GaP crystal material,for example, which has undergone the wet or dry surface treatment forthe removal of an oxide film is to be bonded in a high degree of vacuumto the surface of the outermost surface layer of the light-emitting partwhich has undergone the same surface treatment for the removal of anoxide film, the atomic concentrations of oxygen in their bonding layerscan be decreased to below 1×10²⁰ atoms/cm³ with still betterreproducibility. Alternatively mentioned, the outermost surface layer ofthe light-emitting part and the supporting layer together form a bondinglayer of great bonding strength. Further, by performing the bonding in avacuum, it becomes possible for the necessity of an environment of veryhigh cleanliness needed in effecting the bonding in air to be obviatedand the fabrication to be accomplished stably and inexpensively.

When the outermost surface layer of the light-emitting part and thetransparent supporting layer, besides satisfying the decrease of oxygenconcentration, are bonded in such a manner that carbon (C) or animpurity containing carbon may not persist on their surfaces beingbonded (bonding surfaces), they can be bonded strongly. When thesurfaces to be bonded, after undergoing the surface treatment for theremoval of oxide film, are bonded without undergoing a final cleaningusing an organic solvent, such as methanol (CH₃OH), ethanol (C₂H₅OH) oracetone (CH₃COCH₃), the concentration of carbon atoms in the bondinglayer can be decreased. By not finally subjecting the surfaces beingbonded to a gas phase etching using a carbon-containing substance, suchas carbon tetrachloride (CCl₄), it is made possible for the producedbond to excel in strength.

Particularly, it is enabled to form a strong bond by having theconcentration of carbon atoms in the bonding layer set at 1×10²⁰atoms/cm³ or less. Further, when the outermost surface layer of thelight-emitting part and the transparent supporting layer are bonded in avacuum of high degree of 1×10⁻⁴ Pa or less, for example, a bondpossessing high bonding strength and having the concentration of carbonatoms in the bonding layer set at 1×10¹⁹ atoms/cm³ or less is obtained.By decreasing the atomic concentrations of oxygen and carbon in thebonding layer, it is made possible not only to form a strong bond butalso to suppress the introduction of a crystal defect in the bondinglayer and consequently avoid uncalled-for aggravation of electricresistance in the direction of flow of the electric current foroperating the device (device operating current). As a result, a compoundsemiconductor light-emitting diode (LED) exhibiting a low forwardvoltage (Vf) and a small leak current via a crystal defect and aboundingin reverse voltage can be configured. The atomic concentrations ofoxygen and carbon can be determined by an analytic means, such as theSecondary Ion Mass Spectrometry (SIMS) or the Auger ElectronSpectroscopy (AES).

The transparent supporting layer to be bonded to the outermost surfacelayer of the light-emitting part is formed of an optically transparentmaterial which possesses strength sufficient for mechanically supportingthe light-emitting part and forms a forbidden band wide enough to allowtransmission of the light emitted from the light-emitting part. It canbe formed, for example, of a Group III-V compound semiconductor crystalmaterial, such as gallium phosphide (GaP), aluminum-gallium arsenide(AlGaAs) or gallium nitride (GaN), a Group II-VI compound semiconductorcrystal material, such as zinc sulfide (ZnS) or zinc selenide (ZnSe), ora Group IV compound semiconductor crystal material, such as a hexagonalor cubic silicon carbide (SiC), for example. The transparent supportinglayer preferably has a thickness of about 50 μm or more so that it maybe capable of supporting the light-emitting part with mechanicallysufficient strength. It preferably has a thickness not exceeding about300 μm so that the mechanical work to be performed on the transparentsupporting layer after being bonded may be facilitated. In the compoundsemiconductor LED provided with a light-emitting layer formed of(Al_(X)Ga_(1-X))_(Y)In_(1-Y)P (0≦X≦1, 0<Y≦1), it is most favorable tohave the transparent supporting layer formed of a GaP single crystalmaterial having a thickness of about 50 μm or more and about 300 μm orless.

When a transparent supporting layer formed of gallium phosphide (GaP),for example, is disposed as bonded to the outermost surface layer of thelight-emitting part, the formation of the outermost surface layer of thelight-emitting part with a Group III-V compound semiconductor materialhaving a different lattice constant from the other Group III-V compoundsemiconductor layers constituting the light-emitting part results inmanifesting a function of relaxing the stress exerted on thelight-emitting part while the transparent supporting layer is bondedthereto. As a result, it is made possible to prevent the light-emittinglayer from sustaining damage during the course of bonding and contributeto permission of stable supply of a compound semiconductor LED capableof emitting a light in an expected wavelength, for example. Theoutermost surface layer of the light-emitting part preferably has athickness of 0.5 μm or more in order to sufficiently relax the stressexerted on the light-emitting part while the transparent supportinglayer is being bonded thereto. When the thickness of the outermostsurface layer is unduly increased, the exertion of stress on thelight-emitting layer inevitably occurs while the outermost surface layeris being disposed on account of the difference of the lattice constantfrom the other component layers of the light-emitting part. For thepurpose of avoiding this hardship, the thickness of the outermostsurface layer is preferably set at 20 μm or less.

Particularly, when gallium phosphide (GaP) is selected for thetransparent supporting layer which proves to be convenient for the sakeof transmitting to the exterior the light emitted from thelight-emitting part formed of (Al_(X)Ga_(1-X))_(Y)In_(1-Y)P (0≦X≦1,0<Y≦1), the formation of the outermost surface layer of thelight-emitting part with a semiconductor material having gallium (Ga)and phosphorus (P) as component elements and containing Ga in a largeramount than P enables the bond to be formed strongly. It is particularlypreferable to have the outermost surface layer formed ofGa_(X)P_(1-X)(0.5<X<0.7) of a nonstoichiometric composition.

The surface of the transparent supporting layer and the surface of theoutermost surface layer of the light-emitting part that are ready to bebonded are each formed of a single crystal and are preferably in thesame plane direction. Preferably, the two surfaces both constitute a(001) face. For the purpose of obtaining the outermost surface layer ofthe light-emitting part which has a (001) face for the surface thereofit suffices to use a substrate having a (001) face for the surfacethereof when the outermost surface layer of the light-emitting part isto be bonded on the substrate. By using as the substrate a galliumarsenide (GaAs) single crystal having a (001) face as the surfacethereof, it is made possible to form the outermost surface layer of thelight-emitting part that has a (001) face as the surface thereof.

The light-emitting part can be formed on the surface of a substrate of aGroup III-V compound semiconductor single crystal, such as galliumarsenide (GaAs), indium phosphide (InP) or gallium phosphide (GaP), or asilicon (Si) substrate. The light-emitting part is preferably formed ina double hetero (DH) structure capable of confining the carrier and theemission of light which are responsible for the radiation combination asmentioned previously. The light-emitting layer, with the object ofobtaining light emission excelling in monochromaticity, is preferablyformed in a Single Quantum Well (SQW) structure or a MultiQuantum Well(MQW) structure. As concrete examples of the means for forming thecomponent layers of the light-emitting part, the method for MetalOrganicChemical Vapor Deposition (MOCVD), the method of Molecular Beam Epitaxy(MBE) and the method of Liquid Phase Epitaxy (LPE) may be cited.

Between the substrate and the light-emitting part, a buffer layerresponsible for the action of relaxing the lattice mismatch between thematerial of the substrate and the component layers of the light-emittingpart, a Bragg reflection layer intended to reflect the light emittedfrom the light-emitting layer to the exterior of the device, and anetching stop layer utilized for selective etching are interposed.Further, the component layers of the light-emitting part may be providedthereon with a contact layer for lowering the contact resistance of anohmic electrode, a current diffusion layer for inducing planar diffusionof a device operating current wholly in the light-emitting part, acurrent inhibition layer for conversely restricting the region thatpermits flow of the device operating current, a current constrictionlayer, etc.

By first causing the surface of the transparent supporting layer and thesurface of the outermost surface layer of the light-emitting part whichare ready to be bonded to be equalized in plane direction and thenfurther causing them to form a planar azimuth difference within 20degrees in angle with respect to a specific crystal direction, itbecomes possible to form a strong bond. The surfaces are preferablybonded after they have parallelized (azimuth difference=0 degree) the<110> direction of the gallium phosphide (GaP) single crystal materialused for the transparent supporting layer, for example, and the <110>direction of the nonstoichiometeric composition Ga_(X)P_(1-X)(0.5<X<0.7)forming the outermost surface layer of the light-emitting part, forexample. The fact that the surfaces are so bonded as to avoid occurrenceof a difference in azimuth with respect to a specific crystal directionproves to be most convenient for the purpose of lowering the electricresistance in the interface. If the difference in azimuth on one planeincreases beyond 20 degrees, the excess will result in adding toresistance.

Further, particularly strong bonding can be accomplished when thesurface of the transparent supporting layer or of the outermost surfacelayer of the light-emitting part to which the supporting layer is to bebonded has flatness of 0.3 nm or less in root-mean-square (rms) value.The surface of such flatness as this can be obtained, for example, bythe Chemical Mechanical Polishing (CMP) means that uses an abrasivecontaining silicon carbide (SiC)-based micropowder or micropowder ofcerium (Ce). When the surface polished by the chemical mechanicalpolishing means is further treated with an acid solution or an alkalisolution, the treatment can contribute much to the acquisition of aclean surface by further enhancing the degree of flatness of surface andeffecting removal of a foreign matter or a polluting matter suffered toadhere to the surface during the course of polishing.

The transparent supporting layer or the outermost surface layer of thelight-emitting part is subjected to the bonding in vacuum of 1×10⁻² Paor less and preferably 1×10⁻³ Pa or less in pressure. A particularlystrong bond can be formed by mutually bonding the two flat surfaces thathave been polished as described above. It is important that the twosurfaces, prior to being mutually bonded, be activated by beingindividually irradiated with an atom beam or ion beam possessing energyof 50 eV or more. The term “activation” refers to the creation ofsurfaces in a clean state by depriving the two surfaces being bonded ofan impurity layer or a polluting layer containing an oxide film, carbon,etc. and existing on the surfaces. By performing this irradiation on thesurface of either the transparent supporting layer or the componentlayers of the light-emitting part, the two surfaces are strongly bondedinfallibly. By performing the irradiation on both the surfaces, they canbe bonded with still greater strength.

As concrete examples of the source of irradiation that proves effectivein inducing a strong bond, beams of hydrogen (H) atom, hydrogen molecule(H₂) and hydrogen ion (proton: H⁺) may be cited. By radiating a beamcontaining an element that exists in the surface region about to bebonded, a bond excelling in strength can be formed. When galliumphosphide (GaP) having zinc (Zn) added thereto is to be used for thetransparent supporting layer, for example, the irradiation of thesurface being bonded with an atom or ion beam containing gallium (Ga),phosphorus (P) or zinc (Zn) results in enabling formation of a strongbond. When the surfaces of the transparent supporting layer and theoutermost surface layer of the light-emitting part abound in electricresistance, however, the irradiation of these surfaces with a beammainly containing ions possibly results in electrifying the surfaces.When this electrification of the surfaces causes an electricalrepulsion, the formation of a strong bond cannot be attained. Theactivation of the surfaces by the radiation of an ion beam, therefore,is preferably utilized for the purpose of activating such surfaces asare excellent in electric conductivity.

On the surface region of the transparent supporting layer or thecomponent layers of the light-emitting part, the activation of thesurface can be stably fulfilled by using the beam of an inert gasselected from among helium (He), neon (Ne), argon (Ar) and krypton (Kr)which are incapable of exerting a conspicuous change such as on thecomposition of the surface region. The use of the beam of argon (Ar)atom (mono-atomic molecule) among other inert atoms enumerated aboveproves to be convenient because it is capable of easily activating thesurface in a short time. Helium (He) has a smaller atomic weight thanargon (Ar). The beam of He, therefore, is at a disadvantage in wastingtime while it is being used for activating the surface ready to bebonded. Meanwhile, the use of the beam of krypton (Kr) that has a largeratomic weight than argon proves inconvenient because it is quite capableof inflicting damage of impact on the surface.

When the surfaces of the transparent supporting layer and the outermostsurface layer of the light-emitting part are stacked as opposed to eachother and are consequently readied for bonding, the measure adopted forenabling applied mechanical pressure to cover wholly the surfaces beingbonded proves convenient for strongly bonding both the surfaces. To bespecific, pressure of 5 g·cm⁻² or more and 100 g·cm⁻² or less is appliedin a perpendicular direction to the surfaces being bonded. This methodmanifests the effect of fulfilling the bonding with uniform strengtheven when the transparent supporting layer or the outermost surfacelayer of the light-emitting part or both happen to be warped.

The transparent supporting layer and the light-emitting part are bondedin a vacuum of the preferred degree mentioned above while thetemperature of the surface of the supporting layer or the light-emittingpart or both is set at 100° C. or less, preferably at 50° C. or less,and more preferably at room temperature. When the bonding is performedin an environment of high temperature exceeding about 500° C., thelight-emitting layer formed of (Al_(X)Ga_(1-X))_(Y)In_(1-Y)P (0≦X≦1,0<Y≦1) and provided for the light-emitting part is thermally denatured.This proves to be inconvenient for stably obtaining a compoundsemiconductor LED capable of emitting a light of an expected wavelength.

By bonding the supporting layer to the outermost surface layer of thelight-emitting part, reducing it to a state capable of mechanicallysupporting the light-emitting part, and subsequently removing thesubstrate utilized for forming the light-emitting part, it is madepossible to enhance the efficiency of extraction of the emitted light tothe exterior and permit configuration of a compound semiconductor LED ofhigh luminance. Particularly when an optically nontransparent materialinevitably suffered to absorb the light from the light-emitting layer of(Al_(X)Ga_(1-X))_(Y)In_(1-Y)P (0≦X≦1, 0<Y≦1) is used as the substrate,the means for removing the substrate in such a manner can contributemuch to stable fabrication of an LED of high luminance. When a layersuch as, for example, the buffer layer formed of a material absorbingthe light emitted from the light-emitting part intervenes between thesubstrate and the light-emitting part, the removal of this interveninglayer in conjunction with the substrate proves advantageous forenhancing the luminance of the LED. For the removal of the substrate,mechanical cutting work, abrasion, physical dry or chemical wet etching,etc. are available either singly or in combination. Particularly by theselective etching means that utilizes the difference in etching speedbetween materials, it is made possible to effect selective removal ofthe substrate solely and enable the substrate to be removed uniformlywith high reproducibility.

When the light-emitting part is formed on a substrate made of anoptically transparent and electrically insulating material that isincapable of absorbing the light emitted from the light-emitting layer,the removal of this insulating substrate enables configuration of acompound semiconductor LED of a simple structure which is capable ofpassing the device operating current in the (plumb-bob) verticaldirection. By causing the interface between the insulating substrate andthe semiconductor layer lying directly on the surface of the substrateto be concentrically irradiated with a laser beam, for example, afterthe light-emitting part has been mechanically supported by being bondedto the transparent supporting layer, for example, the substrate isenabled to be peeled off the semiconductor layer lying directly thereon.Then, a unipolar ohmic electrode is formed on the surface of theelectrically conductive semiconductor layer that has been exposed inconsequence of the separation of the substrate. Meantime, an ohmicelectrode of the other polarity is formed on the surface of thesupporting layer that has been bonded so as to support thelight-emitting part. When the ohmic electrodes are disposed in such amanner as described above, since the electrodes of both polarities aredisposed on one surface side and the necessity of removing thelight-emitting part is consequently obviated, the area for emittinglight is increased to the extent of contributing much to the fabricationof a compound semiconductor LED convenient for materializing theemission of light of high luminance.

For example, when the so-called flip chip type diode structure obtainedby causing a transparent substrate to serve as a surface for extractionof light, forming a first electrode on the surface of a semiconductorlayer stripped of the substrate and an electrode on a semiconductorlayer of the second polarity after removing part of the semiconductorlayer, and further disposing a metallic reflection layer adapted tocover the surfaces of the first electrode and the semiconductor layer isenabled to utilize the reflection layer for enhancing the efficiency ofextraction of light, the structure consequently fabricated brings anoptimum advantage. The fabrication of an LED lamp of high luminance canbe accomplished by having a light-emitting diode of this structureincorporated in a package.

EXAMPLE

In the present example, this invention will be specifically described byciting the case of fabricating a light-emitting diode by bonding anepitaxially stacked structure disposed on a GaAs substrate and a GaPsupporting layer.

FIG. 1 and FIG. 2 are schematic diagrams illustrating a semiconductorlight-emitting diode fabricated in the present example; FIG. 1 is a planview thereof and FIG. 2 is a cross section taken through FIG. 1 alongline II-II. FIG. 3 is a schematic diagram of the layer structure of anepitaxially stacked structure used in the semiconductor light-emittingdiode and FIG. 4 is a schematic diagram of the layer structure of theepitaxially stacked structure in a state having a supporting layerbonded thereto.

It is an AlGaInP red light-emitting diode (LED) that was fabricated inthe present example.

An LED (LED chip) 10 shown in FIG. 1 and FIG. 2 was fabricated by usingan epitaxially stacked structure 101 provided with semiconductor layers13 sequentially stacked on a semiconductor substrate 11 formed of anSi-doped n-type GaAs single crystal possessing a surface inclined by 15°from the (100) plane and a p-type GaP substrate (transparent supportinglayer) 14 bonded thereto as shown in FIG. 4.

For a start, such an epitaxially stacked structure 100 as shown in FIG.3 was fabricated. This epitaxially stacked structure 100 was composed ofthe substrate 11 and the stacked semiconductor layers 13.

The stacked semiconductor layers 13 were a buffer layer 130 formed of aTe-doped n-type GaAs, a contact layer 131 formed of a Te-doped n-type(Al_(0.5)Ga_(0.5))_(0.5)In_(0.5)P, a lower clad layer 132 formed of aTe-doped n-type (Al_(0.7)Ga_(0.3))_(0.5)In_(0.5)P, a light-emittinglayer 133 formed of 20 pairs of undoped(Al_(0.2)Ga_(0.8))_(0.5)In_(0.5)P/Al_(0.7)Ga_(0.3))_(0.5)In_(0.5)P, anupper clad layer 134 formed of an Mg-doped p-type(Al_(0.7)Ga_(0.3))_(0.5)In_(0.5)P and an Mg-doped p-type GaP layer 135.The upper clad layer 134 is provided thereon with a thin film formed of(Al_(0.5)Ga_(0.5))_(0.5)In_(0.5)P. A light-emitting part 12 of the LED10 assumes a p-n junction-type double hetero bond structure that iscomposed of the lower clad layer 132, the light-emitting layer 133 andthe upper clad layer 134.

In the present example, the individual semiconductor layers 130 to 135constituted an epitaxial wafer formed as stacked on the GaAs substrate11 by the low-pressure MetalOrganic Chemical Vapor Deposition method(MOCVD method) using trimethyl aluminum ((CH₃)₃Al), trimethyl gallium((CH₃)₃Ga) and trimethyl indium ((CH₃)₃In) as raw materials for GroupIII component elements. As the raw material for the Mg doping,biscyclopentadiethyl magnesium (bis-(C₅H₅)₂Mg) was used. As the rawmaterial for the Te doping, dimethyl tellurium ((CH₃)₂Te) was used.Then, as the raw material for a Group V component element, phosphine(PH₃) or arsine (AsH₃) was used. The GaP layer 135 was grown at 750° C.and the other semiconductor layers 130 to 134 forming the semiconductorlayer 13 were grown at 730° C.

The GaAs buffer layer 130 had a carrier concentration of about 5×10¹⁸cm⁻³ and a layer thickness of about 0.2 μm. The contact layer 131 wasformed of (Al_(0.5)Ga_(O.5))_(0.5)In_(0.5)P and had a carrierconcentration of about 2×10¹⁸ cm⁻³ and a layer thickness of about 1.5μm. The n-clad layer 132 had a carrier concentration of about 8×10¹⁷cm⁻³ and a layer thickness of about 1 μm. The light-emitting layer 133was not doped and had a layer thickness of 0.8 μm. The p-clad layer 134had a carrier concentration of about 2×10¹⁷ cm³ and a layer thickness of1 μm. The GaP layer 135 had a carrier concentration of about 3×10¹⁸ cm⁻³and a layer thickness of 9 μm. The p-type GaP layer 135 on the outermostsurface layer of the light-emitting part 12 had a region reaching adepth of about 1 μm from the first surface polished and subjected tomirror finishing. By the mirror finishing, the surface of the p-type GaPlayer 135 was given roughness of 0.18 nm.

The p-type GaP substrate 14 (FIG. 4) was prepared as a transparentsupporting layer to be affixed to the mirror polished surface of thep-type GaP 135. The GaP substrate 14 ready to be affixed had Zn addedthereto so as to acquire a carrier concentration of about 2×10¹⁸ cm⁻³. Asingle crystal having a plane direction of 15° off (100) was used. TheGaP substrate 14 to be affixed had a diameter of 50 mm and a thicknessof 250 μm. This GaP substrate 14, prior to being bonded to the p-typeGaP layer 135, had the surface thereof mirror-polished and finished till0.12 mn in the rms value.

The GaP substrate 14 and the epitaxially stacked structure 100 werecarried into an ordinary semiconductor material affixing device, and thesemiconductor material affixing device was then evacuated to a vacuum of3×10⁻⁵ Pa. Thereafter, in the semiconductor material affixing devicewhich had excluded members made of carbon materials with a view toavoiding pollution with carbon, the GaP substrate 14 and the epitaxiallystacked structure 100 mounted therein were kept heated to a temperatureof about 800° C. in the vacuum while the surface of the GaP substrate 14was irradiated with Ar ions accelerated to an energy of 800 eV.Consequently, a bonding layer 141 having a nonstoichiometric compositionwas formed on the surfaces of the GaP substrate 14 and the epitaxiallystacked structure 100. Subsequent to the formation of the bonding layer,the radiation of Ar ions was discontinued and the temperature of the GaPsubstrate 14 was lowered to room temperature.

Then, the surfaces of both the GaP substrate 14 possessing in thesurface region the bonding layer 141 made of a nonstoichiometriccomposition and the GaP layer 135 were irradiated over a period of 3minutes with an Ar beam neutralized by bombardment with electrons.Thereafter, in the semiconductor material-affixing device maintained ata vacuum, the surfaces of both the layers 135 and 14 were overlapped asillustrated in FIG. 4 and severally loaded with a pressure till 20g/cm², and mutually bonded at room temperature. When a wafer resultingfrom the bonding was removed from the vacuum chamber of thesemiconductor material affixing device and the interface of bonding inthe wafer was analyzed, the bonding layer 141 formed of anonstoichiometric composition of Ga_(0.6)P_(0.4) was detected in thebonded parts. The bonding layer 141 had a thickness of about 3 nm, theconcentration of oxygen atoms in the bonding layer 141 determined by theordinary method of SIMS analysis was 7×10¹⁸ cm⁻³, and the concentrationof carbon atoms was 9×10¹⁸ cm⁻³.

The back surface of the affixed GaP substrate 14 that lay opposite thesurface of bonding was clad by the ordinary method of vacuum depositionwith a multi-layered film composed of a gold-beryllium (AuBe) alloy filmhaving a thickness of 0.2 μm and an Au film having a thickness of 0.8μm. Then, the multi-layered film was processed by the ordinary means ofphotolithography to form patterns, and the patterns of multi-layeredfilm measuring 50 μm in diameter and assuming a circular shape in theplan view were regularly arrayed like lattice points separated by adistance of 150 μm. Subsequently, these circular patterns ofmulti-layered film were subjected to a heat treatment performed at 450°C. for 10 minutes so as to be alloyed and transformed into a p-typeohmic electrode 16 of low contact resistance.

Then, the GaAs substrate 11 and the GaAs buffer layer 130 wereselectively removed with an ammonia-based etchant. On the exposedsurface of the contact layer 131, an AuGe/Ni alloy film measuring 0.2 μmin thickness and an Au film measuring 0.1 μm in thickness were depositedby the method of vacuum deposition. By the patterning carried out byusing an ordinary means of photolithography, an n-type ohmic electrode15 was formed. Subsequently, the surface of the contact layer 131 andthe surface of the n-type ohmic electrode 15 were coated with anIndium-Tin Oxide (ITO) complex film (transparent, electricallyconductive film) 17 formed in a thickness of 0.5 μm by utilizing anordinary sputtering device. Further, a chromium (Cr) thin film having athickness of 0.03 μm and a gold (Au) thin film having a thickness of 1μm were sequentially stacked on the ITO surface by the sputtering methodand, thereafter, a bonding electrode 18 having a diameter of 110 μm wasformed.

Then, the LED chips 10 of a nearly square shape in the plan view wereformed by crucially inserting cuts at intervals of 250 μm by utilizingan ordinary dicing saw. Incidentally, subsequent to the dicing work, theside faces of the LED chips were etched with a mixed sulfuric acid andhydrogen peroxide liquid for the purpose of removing such shatteredlayers which were fabricated by the cutting on the cut side faces of thesemiconductor layers constituting the epitaxially stacked structure.

The LED chips 10 fabricated as described above were assembled into alight-emitting diode lamp 42 as shown by the schematic diagrams of FIG.5 and FIG. 6. This LED lamp 42 was fabricated by being fixed and mountedon p-electrode terminals 43 disposed on the surface of the mountingsubstrate 45 with silver (Ag) paste and finally sealed with an ordinaryepoxy resin 41, after the bonding electrodes 18 of the LED chips andn-electrode terminals 44 were subjected to wire bonding with a gold wire46. The shear strength exerted on the bonding interface between thesubstrate 45 and the LED chip 10 was about 300 g or more. Since the modeof breaking consisted in the separation of individual LED chips 10, thebonding strength in the bonding interface was interpreted as exceedingthe breaking strength of the crystal layers partaking in the formationof the epitaxial wafer.

The flow of an electric current between the n-type and p-type ohmicelectrodes 15, 16 via the n-electrode terminal 44 and the p-electrodeterminal 43 disposed on the surface of the mounting substrate 45resulted in the emission of a light in red color having a mainwavelength of 620 nm. The forward voltage (Vf) during the flow of anelectric current of 20 mA in the forward direction was found to be about2.2 V, reflecting the lowness of resistance in the bonding interfacebetween the GaP layer 135 and the GaP substrate 14 and the good ohmicproperties of the ohmic electrodes 15 and 16. The intensity of lightemission during the flow in the forward direction of an electric currentfixed at 20 mA was such as to manifest high luminance of 520 mcd,reflecting the fact that the efficiency of extraction of light to theexterior was enhanced as by forming a light-emitting part of highefficiency of light emission and depriving the chips of the shatteredlayers occurring thereon during the course of cutting.

Incidentally, while the ohmic electrode 15 was formed in the presentexample by reason of such a simple construction as illustrated in FIG.1, the electrode may be formed in shapes, such as dots, lattices,circles, squares or their combinations as shown in FIG. 7 to FIG. 11.The selection of the electrode in such a pattern as is suitable fordiffusion of an electric current can contribute to the fabrication of anLED possessing the characteristic properties described in this example.

Comparative Example 1

An AlGaInP LED chip was fabricated by affixing a p-type GaP substratehaving no bonding layer formed on the surface thereof to a p-type GaPlayer under the conditions different from those of the example citedabove. In Comparative Example 1, while the p-type GaP substrate wasmaintained in an atmosphere of nitrogen at a temperature of 800° C. andkept continuously over one hour under such a load as to produce pressureof 200 g/cm² on the bonding surface, the surfaces of the p-type GaPsubstrate and the p-type GaP layer were bonded at an elevatedtemperature under the atmospheric pressure. In the bonding performedunder the conditions described in Comparative Example 1, the presence ofa large amount of crystal defects was detected in the bonding interfacebetween the p-type GaP substrate and the p-type GaP layer.

The concentration of oxygen atoms in the interface region was 2.0×10²⁰cm⁻³ and the concentration of carbon atoms therein was 1.1×10²⁰ cm⁻³,both being high concentrations. Further, the shear strength was as lowas 180 g. Consequently, separation of bond occurred in the regionequivalent to about 10% of the surface area of the bonding surfaceduring the dicing step performed for the formation of chips.

The light-emitting diode chip that tolerated the impact of chippingwithout inducing such separation was tested for the characteristicproperties of an LED. Though the LED emitted a light in red color havinga main wavelength of 620 nm, the forward voltage (Vf) produced duringthe flow of an electric current of 20 mA in the forward directionassumed such a high magnitude as 2.6 V. Conversely, the intensity oflight emission during the flow of a forward electric current fixed at 20mA had a low magnitude of 270 mcd. These results allow an inference thatthe crystal defects inflicted on the bonding interface and the bondingtreatment performed at an elevated temperature during the course ofbonding led to deteriorating the quality of the light-emitting part anddegrading the luminance.

Comparative Example 2

An AlGaInP LED chip was fabricated by affixing a p-type GaP substratehaving no bonding layer formed on the surface thereof to a p-type GaPlayer under the conditions different from those of the example andComparative Example 1 cited above. In Comparative Example 2, the p-typeGaP layer bonded to the p-type GaP substrate had a layer thickness of0.3 μm. This p-type GaP layer, after having the surface thereof rinsedwith hydrofluoric acid (HF), was bonded to the p-type GaP substrate andthey were together subjected to a heat treatment at 500° C. so as tofinish the bonding. The concentration of oxygen atoms in the bondinginterface was 3×10²⁰ cm⁻³ and the concentration of carbon atoms thereinwas 2×10²⁰ cm⁻³, both being high concentrations. The shear strength wasas low as 100 g. Consequently, separation of the bonding surfaceoccurred in the region equivalent to about 40% of the surface area ofthe bonding surface during the dicing step performed for the formationof chips.

The LED emitted a light in red color having a main wavelength of 620 nm.The forward voltage (Vf) during the flow of an electric current of 20 mAin the forward direction was as high as 3.7 V because the resistance inthe bonding interface was high. The intensity of light emission under aforward current fixed at 20 mA was 390 mcd.

INDUSTRIAL APPLICABILITY

The compound semiconductor light-emitting diode can avoid exertion ofstress on the light-emitting part, suppress the occurrence of a crystaldefect, enhance the bonding strength between the light-emitting part andthe supporting layer, further decrease electric resistance in thebonding interface and thereby enhance the forward voltage (Vf), alsoheighten the reverse voltage and materialize impartation of highluminance.

1. A compound semiconductor light-emitting diode comprising: alight-emitting layer formed of aluminum-gallium-indium phosphide((Al_(X)Ga_(1-X)) _(Y)In_(1-Y)P wherein 0 ≦X ≦1, 0 ≦Y ≦1); componentlayers individually having a light-emitting part formed of a Group III-Vcompound semiconductor; a transparent supporting layer bonded to anoutermost surface layer of the light-emitting part and transparent tolight emitted from the light-emitting layer; and a bonding layer formedbetween the supporting layer and the outermost surface layer of thelight-emitting part and containing oxygen atoms at a concentration of 1×10²⁰ cm ⁻³ or less, wherein the outermost surface layer of thelight-emitting part whose side is bonded to the supporting layer and thesupporting layer are both formed of gallium phosphide (GaP), and whereinthe bonding layer has a nonstoichiometric composition represented byformula Ga_(X)P_(l-X) wherein 0.5 <X <0.7.
 2. A compound semiconductorlight-emitting diode according to claim 1, wherein the bonding layerformed between the supporting layer and the outermost surface layer ofthe light-emitting part contains carbon atoms at a concentration of 1×10²⁰ cm⁻³ or less.
 3. A compound semiconductor light-emitting diodeaccording to claim 1, wherein the outermost surface layer of thelight-emitting layer has a different lattice constant from the componentlayers of the light-emitting part and has a thickness of 0.5 μm or moreand 20 μm or less.
 4. A compound semiconductor light-emitting diodeaccording to claim 1, wherein the bonding layer has a thickness of 0.5nm or more and 5 nm or less.
 5. A compound semiconductor light-emittingdiode according to claim 1, wherein a first electrode is formed on theother outermost surface layer of the light-emitting part, a secondelectrode is formed on a surface of the supporting layer, and the firstelectrode is composed of an ohmic electrode formed on the otheroutermost surface layer of the light-emitting part, a transparent,electrically conductive film formed on the ohmic electrode, and abonding electrode is formed on the transparent, electrically conductivefilm.
 6. A method for the fabrication of a compound semiconductorlight-emitting diode comprising: a light-emitting layer formed ofaluminum-gallium-indium phosphide ((Al_(X)Ga_(l-X))_(Y)In_(1-Y)P wherein0≦X ≦1, 0≦Y ≦1); component layers individually having a light-emittingpart formed of a Group III-V compound semiconductor; a transparentsupporting layer bonded to an outermost surface layer of thelight-emitting part and transparent to light emitted from thelight-emitting layer; and a bonding layer formed between the supportinglayer and the outermost surface layer of the light-emitting part andcontaining oxygen atoms at a concentration of 1 ×10²⁰ cm⁻³ or less,wherein the outermost surface layer of the light-emitting part whoseside is bonded to the supporting layer and the supporting layer are bothformed of gallium phosphide (GaP), and wherein the bonding layer has anonstoichiometric composition represented by formula Ga_(X)P_(1-X)wherein 0.5<X <0.7, said method comprising the steps of: growing thecomponent layers of the light-emitting part on a substrate to form thelight-emitting part; polishing the light-emitting part bymirror-polishing the outermost surface of the light-emitting part to anaverage roughness of 0.3 nm or less; preparing the supporting layerseparately of the light-emitting part; irradiating one or both of thepolished outermost surface layer of the light-emitting part and asurface of the supporting layer to be bonded to the light-emitting partin a vacuum with atoms or ions possessing an energy of 50 eV or more;and bonding the polished outermost surface layer of the light-emittingpart and the surface of the supporting layer.